A digital demodulator (such as, but not limited to digital television (TV) demodulators) digitizes an Intermediate Frequency or a base-band signal using an Analog to Digital Converter (ADC). The signal level at the input to the ADC has to be carefully controlled, especially in the presence of high power adjacent channels. If the signal level is too low, quantization noise of the ADC can become too high. If the signal level is too high, clipping or saturation non-linearities can degrade performance.
The signal level at the input of the ADC is controlled using an Automatic Gain Control (AGC) circuit after the ADC. The function of the AGC circuit is to make the average signal amplitude or power level equal to a specified target.
It has been observed that in prior art demodulators setting this target to a fixed level does not optimize performance for all operating conditions. For example, when there is high analog adjacent channel interference (ACI), the target has to be reduced to get best performance. When there is no such ACI, the performance can be improved by increasing the target level.
Note that digital TV or communication systems can have high power ACI for two reasons. Firstly, the transmitted powers of digital channels are kept deliberately low to prevent interference to existing analog receivers. Secondly, the user may be tuned to a digital channel from a distant transmitter while there could be a high power analog or digital channel in the adjacent frequency from a nearby transmitter.
Hence if the AGC is designed with a fixed target, then to cope with high ACI, this target has to be set to a smaller value. However, in most situations this worst case will never occur and hence the demodulator will be operating in sub-optimal conditions.
Thus, a strong need exists for techniques to improve digital demodulator performance.
It will be appreciated that for simplicity and clarity of illustration, elements illustrated in the figures have not necessarily been drawn to scale. For example, the dimensions of some of the elements are exaggerated relative to other elements for clarity. Further, where considered appropriate, reference numerals have been repeated among the figures to indicate corresponding or analogous elements.